There is a continuing need to produce fuels that meet the ever stricter requirements of regulatory agencies around the world. Of particular need are fuels that have relatively low levels of aromatics and sulfur. While regulated fuel properties are not identical for all regions, they are generally achieved by the use of hydroprocessing (hydrotreating) to lower the levels of both aromatics and sulfur. Hydrotreating, particularly hydrodesulfurization, is one of the fundamental processes of the refining and chemical industries. The removal of feed sulfur by conversion to hydrogen sulfide is typically achieved by reaction with hydrogen over non-noble metal sulfides, especially those of Co/Mo, Ni/Mo and Ni/W, at fairly rigorous temperatures and pressures to meet product quality specifications. Environmental considerations and mandates have driven product quality specifications in the direction of lower sulfur and aromatics levels.
Currently, the maximum allowable sulfur level for U.S. on-road diesel is 500 wppm. All countries in the European Community have instituted maximum sulfur levels of 500 wppm. In some European countries diesel fuels having even lower sulfur levels are produced. For example, Swedish Class I and Class II diesel fuels currently allow maximum sulfur levels of 10 and 50 wppm, respectively. It seems very likely that other European countries will move to the &lt;500 wppm sulfur fuels in the foreseeable future.
Environmental and regulatory initiatives are also requiring lower levels of total aromatics in hydrocarbons and, more specifically, lower levels of the multi-ring aromatics found in distillate fuels and heavier hydrocarbon products (i.e., lubes). The maximum allowable aromatics level for U.S. on-road diesel, California Air Resources Board (CARB) reference diesel and Swedish Class I diesel are 35, 10 and 5 vol. %, respectively. Further, the CARB reference diesel and Swedish Class I diesel fuels allow no more than 1.4 and 0.02 vol. % polyaromatics, respectively.
During hydrotreating, aromatics are saturated and feed sulfur is converted to hydrogen sulfide. While this achieves the desired result with respect to emissions, it has an adverse affect on the inherent lubricity properties of the distillate fuel. This lower lubricity leads to increased maintenance costs of diesel engines, e.g., pump failures, and in extreme cases to catastrophic failure of the engine. Consequently, there is a need in the art for processes that can produce distillate fuels that meet current emissions requirements with regard to low aromatics and sulfur, but which have good inherent lubricity properties.